This invention relates to a catalytic process for hydroprocessing of heavy processing streams containing asphaltic materials, metals, sulfur-containing compounds, and nitrogen-containing compounds. More particularly, this invention relates to a hydroprocessing catalyst having improved activities in the desulfurization, demetallation, and denitrogenation of heavy hydrocarbon streams.
Decreasing supplies of high quality crude has focused considerable attention on refining lower quality hydrocarbon feedstocks in recent years. It is widely known that various organo-metallic compounds and asphaltenes are present in petroleum crude oils and other heavy petroleum hydrocarbon streams, such as petroleum hydrocarbon residua, both atmospheric and vacuum residua, oils obtained from tar sands and residua derived from tar sand oil, and hydrocarbon streams derived from coal. These hydrocarbon feedstocks contain organo-metallic contaminants which create deleterious effects in various refining processes that employ catalysts in the conversion of heavy hydrocarbon feedstocks.
The most common contaminant metals found in heavy hydrocarbon streams are nickel, vanadium, and iron. The nickel is present in the form of soluble organo-metallic compounds in concentrations of about 20 ppm to 500 ppm. The presence of nickel porphyrin complexes and other organo-nickel complexes causes severe difficulties in the refining and utilization of heavy hydrocarbon fractions, even if the concentration of these complexes is very small. For instance, it is known that a cracking catalyst deteriorates rapidly and its selectivity changes when in the presence of an appreciable amount of organo-nickel compounds. The deposition of nickel compounds in the interstices between catalyst particles can cause deactivation of the catalyst and plugging which leads to a significant pressure drop in fixed bed reactors. Iron-containing and vanadium-containing compounds are present in practically all crude oils that are associated with the high Conradson carbon asphaltenic portion of the crude. Of course, these metals are concentrated in the residual bottoms when a crude is topped to remove fractions that boil below about 450.degree. F. to 600.degree. F. If such residuum is treated by additional processes such as fluid catalytic cracking, the presence of these metals as well as sulfur and nitrogen can adversely affect the catalyst used in these processes. Further, if an oil containing these metals is used as fuel, the metals will cause poor fuel oil performance in industrial furnaces since they corrode the metal from which the furnace was constructed.
While metallic contaminants, such as vanadium, nickel, and iron, are often present in various hydrocarbon streams, other metals are also present in hydrocarbon feedstocks. These metals exist as oxides or sulfides of the particular metal, or as a soluble salt of the metal, or as high molecular weight organo-metallic compounds, including metal naphthenates and metal prophyrins, and derivatives thereof.
Another problem associated with the conversion of heavy hydrocarbon feedstocks is the formation of insoluble carbonaceous substances known as Shell hot filtration solids from the asphaltenic and resin fractions of the feedstock. Although not wishing to be bound by theory, it is believed that these solids are formed when the heavy hydrocarbons are converted more rapidly in the hydroconversion unit to lower molecular weight oils, thereby rendering them a poorer solvent for the unconverted asphaltenic fraction and hence creating these solids.
Shell hot filtration solids can cause operability problems in hydroprocessing units. In high concentrations, these solids accumulate in lines and separators, causing fouling and in some cases interruption or loss of process flow. The formation of these solids results in the agglomeration of the catalyst, thereby causing high pressure drops through fixed catalyst beds. In an ebullated bed type reactor, catalyst agglomeration can prevent proper mixing of the oil, hydrogen, and catalyst. This allows for uncontrolled reactions and local hot spots that can result in failure, fires, or explosions. Further, the higher the conversion level for given feedstocks the greater the amount of Shell Hot Filtration solids formed.
In the past, refiners have limited their use of feedstocks characterized by formation of Shell hot filtration solids, or in the alternative, limited the conversion of such feedstocks.
Treating the above-described heavy hydrocarbon feedstocks to remove unwanted contaminant metals and poisons can be achieved generally by contacting with hydrogen in the presence of a catalyst under conditions that vary somewhat depending on factors such as the particular feed to be upgraded, the type of process being operated, reaction zone capacity, and other factors known to those skilled in the art. The catalyst typically comprises a hydrogenation component deposited on a porous, inorganic, oxide support. Typical catalyst properties included surface areas of about 50-400 m.sup.2 /g, pore radii of about 10-300 Angstroms, and total pore volumes of about 0.1-20 cc/g.
The key catalyst properties that can be varied to optimize a hydroprocessing catalyst for removal of certain contaminant metals and poisons from certain heavy hydrocarbon feedstocks include catalytic metal loadings, surface area, bulk density, pore volume, and pore radius. The interrelationships between these properties can have various effects on catalyst performance. For example, the desirability of maximizing the surface area in order to provide high exposure of feedstock components to catalytically active sites, and thus maximum activity is well known. At the same time, however, if surface area is too high, bulk density and mechanical strength can decrease to the point that use of the catalyst to remove contaminant metals and poisons from heavy feedstock is impractical or even impossible despite high activity.
Subject to the aforesaid considerations with respect to bulk density and crush strength, it is desirable to provide catalyst particles having a high level of small or intermediate-sized pores because, for a given total pore volume, distribution thereof in smaller pores gives higher surface area than distribution in a smaller number of larger pores. While smaller pores are desirable from the standpoint that they have the highest activities for denitrogenation and desulfurization through maximizing surface area, such pores are also more susceptible to rapid deactivation because pore mouths are quickly blocked by the relatively large species present in heavy hydrocarbon feedstocks. Thus, if too many pores of too small sizes are present, then demetallation activity often declines substantially during process use. If activity declines too rapidly, losses in productivity and increases in catalyst replacement costs can occur.
In the past, bimodal, hydroprocessing catalysts have been developed to strike a balance between catalyst physical properties. Bimodal is defined for the purposes of this application as any catalyst that has at least about 0.05 cc/g of pore volume in pores that are less than about 600 Angstroms radius and at least about 0.05 cc/g of pore volume in pores that are greater than about 600 Angstroms radius. Bimodal catalysts can differ as to surface area, average pore size, and how the the pore volume is distributed throughout the pores.
U.S. Pat. No. 4,454,026 (Hensley, Jr. et al.) discloses a bimodal catalyst characterized by a surface area of 150-190 m.sup.2 /g and a total pore volume of 0.9-1.5 cc/g. This catalyst includes 0.9-1.2 cc/g in pores less than 600 Angstroms radius and 0.15-0.50 cc/g in pores greater than 600 Angstroms radius. U.S. Pat. No. 4,707,466 (Beaton et al.) discloses a hydroprocessing process for hydrocarbon feedstocks containing asphaltenes, metals, and sulfur compounds. This process uses a bimodal catalyst characterized by a surface area of less than 220 m.sup.2 /g and a total pore volume of 0.85-1.5 cc/g. Out of that total pore volume, 0.15-0.40 cc/g is in pore having a radius greater than 600 Angstroms radius. While both of these bimodal catalysts have excellent properties, it is desirable to provide higher activity catalyst.
U.S. Pat. No. 4,225,421 (Hensley, Jr. et al.) discloses a process for hydrodemetallation and hydrodesulfurization of hydrocarbon feedstocks containing asphaltenes and metals by contacting the feedstock with hydrogen and a bimodal catalyst consisting essentially of a Group VIB hydrogenation metal on a support comprising alumina wherein said catalyst has a surface area of about 140-300 m.sup.2 /g and a total pore volume of about 0.4-1.0 cc/g. Out of that total pore volume, 0.06-0.3 cc/g can be in pores having a radius greater than 600 Angstroms. U.S. Pat. No. 4,746,419 (Peck et al.) discloses a bimodal catalyst characterized by a surface area of 75-400 m.sup.2 /g and a total pore volume of 0.5-1.5 cc/g. Out of that total pore volume, 0.2-0.3 cc/g are in pores greater than 600 Angstroms radius and not more than 0.15 cc/g in pores greater than 2000 Angstroms radius. Although Hensley, Jr. et al. and Peck et al. broadly disclose that high surface area, bimodal catalyst are desirable for hydroprocessing heavy hydrocarbon feedstocks, these patents fail to appreciate that the manner in which the pore volume is distributed throughout the range of pores that are less than 600 Angstroms radius can make a difference in catalyst performance, particularly when the hydrocarbon feedstock contains Shell hot filtration solids. In addition, the Peck et al. patent suggests that pores less than 600 Angstroms radius (mesopores) are irrelevant to the production of Shell hot filtration solids.
Applicants have discovered that a high surface area, bimodal catalyst characterized by the prescribed distribution of incremental pore volume throughout a critical range of pores less than about 600 Angstroms radius unexpectedly performs better than bimodal catalysts that have distributions of incremental pore volume that fall outside the critical pore size range. Applicants will show in the upcoming examples that catalysts that have physical properties such as surface area and pore volumes which fall within the broad teaching of Peck et al. perform differently depending upon whether their incremental pore volume distribution falls outside or inside of the critical range of pore sizes.
It is a general objective of the present invention to balance catalyst physical properties to provide a catalyst that admits a maximum amount of treatable heavy hydrocarbons into the pores of the catalyst while at the same time providing access to a maximum number of active catalytic sites.
It is a general objective of the present invention to provide a catalyst and process affording superior demetallation, dehydrosulfurization, denitrogenation, and hydroconversion of hydrocarbon feedstocks containing metals, sulfur, nitrogen, and Shell hot filtration solids precursors.
It is another general object of the present invention to provide a catalyst and process affording a higher conversion level or higher throughput for heavy hydrocarbon feedstocks that tend to form greater amounts of Shell hot filtration solids, in particular that fraction of the feedstock that boils over 1000.degree. F.
Another object of the present invention is to provide a catalyst and process that produces effluent distillates having lower nitrogen and sulfur contents affording such distillates higher values in subsequent blending processes.